| TECH-TIP: Ensure the Accuracy of pH Measurement
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TECH-TIP: Ensure the Accuracy of pH Measurement

Although it is not a parameter directly reportable to the Partnership for Safe Water, pH is an important water quality and process control parameter for both the treatment plant and distribution system.  The pH of the water is a parameter that can impact the effectiveness of many water treatment processes, including coagulation, disinfection and disinfectant by-product formation, and softening, as well as the water’s corrosivity, with pH included among the calculations for several major corrosion indices.

This, in turn, can impact the effectiveness of particulate removal through the treatment plant, the ability to maintain a disinfectant residual, DBP compliance, and distribution system corrosion.  Water treatment processes employed at the plant may also impact pH – for example, some common coagulant chemicals can consume alkalinity and reduce pH, particularly in low alkalinity waters.  Because of this, the impact of pH, and other water quality parameters, must be considered when evaluating treatment or source water changes that may affect a utility's ability to simultaneously comply with multiple regulations.

Due to its potential to influence water treatment process performance, pH is frequently monitored throughout the plant, from the source water to the finished water, and in the distribution system, using grab samples or continuous monitors that provide real-time water quality data.  Data from pH measurements may be used for many purposes.  It can help operators maintain awareness of changes in raw water quality conditions so that treatment may be adjusted accordingly.  Plants that have the capability to feed chemicals for pH adjustment will also monitor pH to control this process and maintain pH at the ideal levels for optimized performance.

Although pH is a common water quality measurement, pH measurement can also present some challenges.  To address some of the common challenges, the Partnership for Safe Water presents the following tips for pH measurement:

Calibration – pH meters and probes typically require calibration on a regular, even daily, basis.  Buffer solutions are used to calibration pH meters, and the most common buffers typically used are at pH 4, 7, and 10.  A two or three point calibration should be used to calibrate the meter, with the buffers selected based on the expected pH of the samples to be measured.  At least one of the buffers used for calibration should be the pH 7 buffer.  Why is this important?  Remember that pH is a logarithmic scale, meaning that every change in one pH unit is a ten-fold change in the water’s hydrogen ion concentration.  The difference between pH 4 and 10 is actually a one million times difference in hydrogen ion concentration!  The effectiveness of calibration can be verified by checking the meter’s calibration slope.  A slope close to 100% or -59.2 mV/pH unit is considered optimal.  If the calibration slope is low, check that the buffers used for calibration were freshly prepared.  Although all buffers have a shelf life, the shelf life of a pH 10 buffer is typically the shortest, due to its tendency to adsorb carbon dioxide from the air.  This forms carbonic acid in solution, which can reduce the pH of the pH 10 buffer and impact the accuracy of calibration.

Verification – When measuring pH, it is recommended to periodically verify the accuracy of measurements.  This can be accomplished by periodically measuring a buffer solution and comparing the reading to the expected reading (which it typically provided with the buffer solution) to ensure its accuracy.  If the pH readings are outside of the expected tolerances, it may be necessary to recalibrate the meter or clean the probe.  When performing a verification reading, take care to perform the measurement under the same conditions used to measure routine samples.  For example, if samples are gently stirred when measured, which is recommended in many cases, the verification check sample should be similarly stirred.

Cleaning and Maintenance – Although pH measurement technology is continually evolving, most pH probes still consist of a glass electrode for measurement.  After frequent use, buildup can deposit on the probe, which can result in slow response and/or inaccurate readings.  Cleaning typically consists of soaking in a variety of solutions, such as dilute acid, base, or a mild soap solution.  The cleaning procedure is dependent on the probe construction and nature of the contamination.  In addition to cleaning, some pH probes will require maintenance, such as refilling the electrolyte filling solution or replacing a salt bridge.  Since pH probes can vary significantly from one manufacturer to another, it is very important to follow the manufacturer’s instructions for cleaning and maintenance procedures. Also note that pH probes have a finite lifespan, and will need to be replaced, at some point, to maintain performance.

Measurement – Samples that are measure in a laboratory setting are typically measured with a meter and a probe. Typically samples are placed on a stir plate, with the sample mixed gently using a stir bar (do not generate a vortex, which draws in air and can potentially affect the sample’s pH).  It is recommended to be as consistent as possible in procedures when measuring samples.  Between samples, the probe should be rinsed with deionized water and gently blotted dry with a clean laboratory wipe, in order to prevent contamination from one sample to the next.  Samples that are low in conductivity (low ionic strength samples) can present a unique measurement challenges, as these readings can be particularly slow to stabilize.  In these cases, procedures are available to condition the electrode to low ionic strength conditions prior to measurement.  As stated above, always follow the manufacturer’s instructions with regard to sample measurement, calibration, and electrode conditioning procedures for best results.

Since pH is such an important parameter to maintaining optimal water treatment plant performance, it is important that pH measurements are as accurate as possible.  The tips provided in this article can help, but there are many resources available that operators can turn to for additional information, such as AWWA’s M12 Manual (Simplified Procedures for Water Examination, 6th Edition) and Standard Methods for the Examination of Water and Wastewater.  By following the pH measurement steps provided in these resources, along with instructions provided by the equipment manufacturers, operators can know that they are taking proactive steps to ensure the accuracy of this important water treatment and distribution system water quality parameter.

Article contributed by Barb Martin, Partnership Sr. Manager
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